Syrian security source said they had targeted a powerful coalition of Islamist rebels which includes Al-Qaeda's Syria affiliate and which is fiercely opposed to IS.

"Air strikes from four Russian warplanes struck bases held by the Army of Conquest in Jisr al-Shughur and Jabal al-Zawiya in Idlib province," the source said, adding that arms depots held by "armed groups" in neighbouring Hama province were also targeted.

A member of the Army of Conquest, which controls Idlib province and has advanced west towards Assad's coastal heartland of Latakia, said on Twitter that "Russian pigs" had flattened a mosque in Jisr al-Shughur.

In Moscow, the defence ministry said it had bombed "four Islamic State targets" in Syria overnight, destroying "the headquarters of terrorist groups and a weapons warehouse in Idlib area and a command centre ... in Hama region".

A car bomb factory north of Homs was also destroyed, it said.

On Wednesday, Russian Foreign Minister Sergei Lavrov brushed off doubts by US officials that Russia was attacking IS targets.

"The rumours that the target of these air strikes was not IS positions are unfounded," Lavrov told journalists in New York after meeting his US counterpart, John Kerry.

He asked the US to provide evidence "because we stand by our targets".

Russia's defence ministry said fighter jets had carried out 20 sorties on Wednesday, striking "eight Islamic State targets" including a command post in the mountains.

Washington complained that Moscow gave only an hour's notice of the strikes, but the two sides were preparing to hold military talks on the situation, perhaps as soon as Thursday.

"We agreed on the imperative of as soon as possible - perhaps even as soon as tomorrow, but as soon as possible - having a military to military de-confliction discussion," Kerry said on Wednesday, appearing with Lavrov on the sidelines of the UN General Assembly.

It remains unclear how much of the opposition fighting Assad's army, including the Western-backed moderate opposition, is considered by Moscow as a potential target.

Speaking to Russian news agencies late on Wednesday, President Vladimir Putin's spokesman, Dmitry Peskov, said Moscow was helping Syria fight IS and "other terrorist and extremist groups".

When asked whether the Free Syrian Army, the Western-backed moderate rebels fighting both Assad and IS, is on Moscow's list of terrorist groups, Peskov said: "Does it exist, the Free Syria Army? Haven't most of them switched to IS group?

"What is the Free Syrian Army, is it an official term? ... It existed but whether it does now nobody knows for sure, it's a relative concept."

Russia and the West are in deep disagreement over Syria, with Moscow backing Assad while Western powers blame him for starting what has become a brutal war with more than 240,000 people dead and millions displaced.

France on Wednesday launched an inquiry into Assad for alleged "crimes against humanity" including kidnappings and torture.

Moscow, however, has portrayed Assad as the only force stopping the spread of IS and argues that he must be part of the political solution to the conflict.

The Russian state television channel Rossiya said Thursday Syria's military is "launching an offensive in northern Homs where over 5000 militants are hiding".